The presence of large deposits of oil shale in the semi-arid, high plateau region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products, including hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein; the carbonaceous liquid product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either mining the kerogen-bearing shale and processing the shale on the surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale, after kerogen decomposition, contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is explosively expanded toward one or more voids excavated in the subterranean formation to form a fragmented permeable mass of formation particles containing oil shale in the retort. The fragmented mass of particles is ignited near the top of the retort to establish a combustion zone. An oxygen-supplying gas is introduced into the top of the retort to sustain the combustion zone and cause it to move downwardly through the fragmented mass. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to release shale oil and gaseous products therefrom in a retorting zone. The retorting zone moves from the top to the bottom of the retort ahead of the combustion zone and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal. Recovery of liquid and gaseous products from oil shale deposits is described in greater detail in U.S. Pat. No. 3,661,423 to Donald E. Garrett which is incorporated herein by this reference.
As used herein, the term "retorting zone" refers to that portion of the retort where kerogen in oil shale is being decomposed to liquid and gaseous products, leaving residual carbonaceous material in the retorted oil shale. The term "combustion zone" refers to a portion of the retort where the greater part of the oxygen in the retort inlet mixture that reacts with the residual carbonaceous material in the retorted oil shale is consumed.
There are several mining techniques that can be used for forming an in situ oil shale retort. One such technique includes excavating one or more generally horizontally extending box-shaped voids into the formation. The box-shaped voids are spaced vertically apart from each other by zones of unfragmented formation that extend between the voids. The zones of unfragmented formation provide generally horizontal free faces above and below the voids towards which the formation is explosively expanded to form the fragmented mass of formation particles in the retort. Pillars of unfragmented formation (hereinafter called "support pillars") can remain in the voids to support overlying formation during excavation, drilling and blasthole loading operations. Such support pillars are explosively expanded into the voids just prior to the explosive expansion of the zones of unfragmented formation.
Information regarding techniques for explosively expanding zones of unfragmented formation toward horizontal free faces for forming a fragmented mass of formation particles in a retort, including techniques for explosively expanding support pillars, can be found in my U.S. Pat. No. 4,300,800. U.S. Pat. No. 4,300,800 is incorporated herein by this reference.
When unfragmented formation is explosively expanded, e.g., toward a void space when forming a retort, it increases in bulk due to void spaces in interstices between the particles. The maximum expansion of an oil shale formation into an unlimited void results in a fragmented mass of oil shale particles having an average void fraction of about 35 percent; that is, about 35 percent of the total volume occupied by the fragmented mass is void space between the particles. The volume occupied by the fragmented mass is about 55 percent larger than the volume occupied by the original unfragmented formation after such unlimited or free expansion.
A "limited void" is one where the void space available for explosive expansion is less than needed for free bulking of the formation expanded toward that void. Thus, if a void has an excavated volume less than about 35 percent of the total of the volume of the void plus the volume occupied by formation explosively expanded, it is necessarily a limited void. It has been found that factors in addition to total available void can make a void "limited" even though the total available void may appear sufficient for free bulking.
When oil shale is explosively expanded toward a limited void, the void fraction of the fragmented mass of particles formed can be no more than permitted by the available void space of the void and, in some instances, has been found to be less. It is believed that the void fraction of the fragmented mass can be less than the available void space provided by such a limited void because when oil shale is explosively expanded toward the void, gases from the detonation may not have full opportunity to act on the oil shale particles before such particles reach obstructions, such as adjacent walls, a face opposite to the expanding formation, or oil shale expanding from the opposite sides of the void.
Thus, when formation is expanded toward one or more limited voids for forming an in situ oil shale retort, a void space can remain in the completed retort between the top surface of the fragmented mass of particles formed and overlying unfragmented formation, i.e. the retort top boundary.
During the retort ignition process, hot oxygen supplying gases are directed into the void space or plenum at the top of the retort to contact the top surface of the fragmented mass and ignite it. So that the ignition of the fragmented mass can be uniform, it is important that no material falls from the retort top boundary into the void space. For example, material that falls from the top boundary can prevent ignition gases from contacting and igniting the fragmented mass surface on which the fallen material rests. This results in a non-uniform combustion zone and can reduce the yield of products from the retorting operation. Additionally, such material can inhibit uniform gas flow through the fragmented mass during retorting operations by changing gas flow patterns in the plenum. Such non-uniform gas flow can result in inefficiencies in the retorting operation thereby reducing product yield.
When an in situ oil shale retort is formed using the above described horizontal free face blasting technique, it can be desired that the roof of the uppermost void excavated into the formation be the top boundary of the retort. Thus, the roof of such a void overlies the fragmented mass of formation particles that is eventually formed within the retort boundaries. Since the roof of such a void is the top boundary of the retort and it is important that no material falls from the top boundary into the void, it is desired that the roof is not structurally damaged during the support pillar blasting operation. For example, in one prior horizontal free face blasting operation used for forming an in situ retort where a pair of support pillars was explosively expanded into the uppermost void, large slabs and blocks of formation from the retort top boundary fell onto the top surface of the fragmented mass in the completed retort. Since the locus of these slabs and blocks was at the original locus of the two support pillars and the section of roof between the pillars, it is thought that detonation of the explosive charges in the pillars damaged the unfragmented formation between and above them sufficiently to cause the massive rock fall.
Additionally, it is important that the zones of unfragmented formation located above and/or below such a support pillar that are to be explosively expanded for forming the fragmented mass in the retort are not fractured or damaged by the pillar blasting operation. Such damage can result in the fragmented mass formed by explosive expansion of such zones of unfragmented formation having a non-uniform particle size distribution and/or a non-uniformly distributed void fraction.
Furthermore, when such support pillars are located in the lowermost void space in the subterranean formation blasting such pillars can fracture or weaken underlying formation; for example formation adjacent product withdrawal drifts. This can result in collapse of material into the withdrawal drifts thus hindering product recovery operations.
Therefore, there is a need in the art for a support pillar blasting method that minimizes blast damage to formation adjacent the pillars.